This invention relates generally to thermal inkjet printing. More particularly, this invention relates to a novel printhead design for increasing the number of inkier nozzles on the printhead while minimizing the number of electrical interconnections between the printer and printhead. The term "printhead" as used herein includes an ink source as well as an ink drop generating mechanism attached to the source and is also known as a cartridge or pen.
Print quality and throughput are important objectives in the design of thermal inkjet printers. Print quality is a function of, among other things, the distance between the inkjet nozzles on the printhead. Higher print quality produces crisper output and more colors through better dithering. Throughput is a function of the width of the printed swath. The wider the swath, the fewer passes the printhead make to print a page.
Both of these objectives can be met by increasing the number of inkjet nozzles on the printhead. By placing nozzles closer together, the print quality can be improved. By placing more nozzles on the printhead, the width of the printing swath is increased. However, adding nozzles requires adding associated drivers, which comprise heater resistors, control logic and power and control interconnections. These interconnections are flexible wires or equivalent conductors that electrically connect the drivers on the printhead to printhead interface circuitry in the printer. They may be contained in a ribbon cable that connects on one end to control circuitry within the printer and on the other end to driver circuitry on the printhead.
Interconnections are a major source of cost in printer design, and adding them to increase the number of drivers increases the cost. Interconnections also affect the reliability of the printer, with more interconnections increasing the likelihood that the printer will fail. Thus as the number of drivers on a printhead has increased over the years, there have been attempts to reduce the number of interconnections per driver. One approach that is presently being investigated is called "integrated drive head" or IDH multiplexing. In IDH, the drivers are split into groups known as primitives. Each primitive has its own power supply interconnection ("primitive select") and return interconnection ("primitive return"). In addition, a number of control lines are used to enable particular drivers. These control or address lines are shared among all primitives. This approach can be thought of as an XY matrix where X is the number of primitives (rows) and Y is the number of drivers per primitive (columns). The energizing ("firing") of each driver resistor is controlled by a primitive select and by a transistor such as a MOSFET that acts as a switch connected in series with each resistor. By powering up one or more primitive selects (X1, X3, etc.) and driving the associated gate of the transistor (Y2, for example), multiple heater resistors may be fired simultaneously. The number of interconnections required for such a matrix is fewer than one per driver. This is markedly fewer than in a direct drive approach, wherein each driver has its own primitive select and shares a primitive common with the other drivers.
The matrix approach in IDH multiplexing offers an improvement over the direct drive approach. Yet as presently contemplated the matrix approach has its drawbacks. The primitive select interconnection for each primitive must be driven by a switching power supply that can rapidly switch between on and off states. Such supplies are more expensive and more prone to failure than constant, i.e., static, power supplies. Furthermore, the number of interconnections with such a matrix is still large, on the order of 3.sqroot.n, where n is the number of drivers. Thus increasing the number of drivers significantly, even with the matrix, still results in an undesirable increase in the number of interconnections.